Yes. Stop all other lines of debugging and focus on this issue. Why
is there an error there? Can you log in as that user?

laltest@lal-squeeze:~$ su -l laltest

No, as this account is a special account without a password. We can
only log into this account using a shared ssh key.

If not then why not? Start debugging there.

Also I have a cron job scheduled, via crontab, to run at 21:01
everyday. It fails to run and the only reference to this I can find in
the logs is the following:
Apr 8 21:01:01 lal-squeeze CRON[14107]: Authentication failure
So I imagine that this is a different manifestation of the same problem.

Everything points to the same problem. Debug it first.

Any recommendations on how to debug this issue?

Look in /var/log/syslog for messages. Look in /var/log/auth.log for
messages.

I can't see anything in the logs around the time that I try and
connect to the box.

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